Why must you drive slowly now, and be ready to brake?

Your answer is correct: the main reason you must drive slowly and be ready to brake is because of the pedestrian. According to German traffic rules, drivers must always be especially cautious and reduce their speed near pedestrians, since they can enter the road unexpectedly. In such situations, being prepared to brake is essential for safety. The other answers do not apply here: the bus is stationary and does not have its hazard lights on, so you are not required to slow to walking pace. There is no cyclist present, so that answer is also not relevant. In general, always focus on pedestrians whenever they are near the roadway—they always have priority and may behave unpredictably.